ALPECOLE
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Snow and perennial ice

Flow and temperature


 

Flow

glacier surge

In addition to advancing and retreating, glaciers actually flow. The curving of crevasses and banded structures known as ogives, the recordable movement of surface rocks over a period of days, and the occasional cracking and creaking sounds within the ice are all symptoms of this. Ice flow is also indicated by the eroded rocks and deposits that are left behind after the ice margins have retreated. The nature of the ice flow, in fact, very much determines the character of a glacier.

Rates of movement of flowing glaciers are extremely variable. Some small glaciers and ice caps may flow only a few metres a year, or are motionless. The fastest parts of most reasonably sized valley glaciers flow anywhere between 50 to 400 metres a year. Large ice streams in Antarctica and outlet glaciers in Greenland even flow at a rate of a thousand metres a year or more. Some glaciers flow in a rather unpredictable manner, remaining relatively inactive for many years, then accelerating suddenly. For a period of a few months they advance over distances measured in kilometres. This phenomena is called glacier surge.

1 - Glacier surge, Rio Plomo, Argentina, notice the heavily crevassed surface of the glacier (120K)

Ice flows by two main processes:

  1. internal deformation and
  2. sliding on its bed

 

Internal deformation

As snow turns to firn and then ice, its constituent crystals alter under the weight of material accumulating above them and under the influence of gravity. These stresses cause the ice to change shape in a rather plastic manner, much as soft putty or porridge will deform on a slope, only considerably slower.

A typical flow pattern shows an initial, rapid increase in velocity away from the margins, then a declining rate of increase towards the middle.
Such a velocity profile is parabolic in shape. Similarly, there is a rapid increase in velocity in the first several metres above the bed of a glacier (see figure 2) , then the rate of flow remains essentially constant through the rest.

 

verlocity and depth

2 - Variation of horizontal velocity with depth, Athabasca glacier, Canada. Data from Savage and Paterson (1963) in Paterson (1994)

Basal Sliding

The second component of glacier flow is basal sliding, whereby the glacier slips over its bed. Large quantities of melt water produced in summer reduce the friction between a glacier and its bed and cause faster flow.

In a temperate glacier, basal sliding is the major component of its flow, and may account for as much as 90% of its total movement. Where basal sliding occurs over uneven bedrock it often generates caves, where processes of erosion and deposition can be studied. Since sliding velocities are related to the amount of meltwater available, glaciers move

  • faster in summer than in winter, and
  • faster in daytime than at night

 

Exceptional speeds may also be induced by heavy rain. In cold glaciers basal sliding can only occur where the ice is thick enough for the base to be warmed to melting point. Moreover, it is quite common for the snouts of otherwise sliding glaciers to be frozen to the bed because the ice is thinner there and thus affected by the low mean annual air temperature. In addition, a layer of unconsolidated sediment known as till often underlies moving ice. This is a mixture of particles of all sizes from clay to boulders. When this material is saturated with water, this sediment deforms more easily than the basal ice, and glacier movement is assisted by shearing within the soft, deformable sediments, rather than by sliding.


These time-lapsed photographs (532K) show the increase in the snow line from June to August. In addition, the flow of the glacier ice can be very easily recognised, especially from the high ice velocities in the ice fall from the perennial snow field (Ewigschneefeld, upper right side of the picture).
Source: http://www.webcam24.ch/d/channel3/konkordiahuette/index.shtml

konkordia_huette

3 - A time-lapse webcam photo of the Konkordiahuette at 2850m (532K)

 


Temperature

The temperature distribution in glaciers and ice sheets deserves added attention because of its influence on different processes. The present variation of temperature with depth provides information about past variations of surface temperature. The deformation rate of ice is very sensitive to temperature; cooling from –10°C to –25°C reduces this rate by a factor of five. If the temperature of a glacier, previously frozen to the ground, were to reach the melting point at ground level, the ice could start to slide and lead to unstable behaviour of the glacier. The basal temperature also controls erosion; the bed is protected if the ice is frozen to it. Properties such as the velocity of seismic waves, the absorbtion of radio waves and the DC resistivity, on which methods of measuring ice thickness depend, also vary with temperature. The distribution of temperature in the ice is mainly controlled by the surface temperature and therefore by the energy balance. In addition, if the ice slides, geothermal heat and friction warm or melt the base while ice deformation and in some cases refreezing of meltwater warm the interior. Conduction, ice movement, and in some cases water flow transfer heat within the glacier.
Based on the temperature distribution in the glacier, three main types have been distinguished:

Cold glaciers

Polythermal glaciers

Temperate glaciers

cold glacier polythermal glacier temperate glacier

4 - Cold glacier are cold throughout their whole ice mass. (84K)

5 - Polythermal glaciers show a mixed temperature distribution; some parts are cold and others are temperate. (112K)

6 - Temperate glaciers are at the melting point throughout. (84K)


firn temperature englacial temperatures

7 - Temperatures measured at 18m in the Monte Rosa area showing grid-interpolated values. Black points represent the borehole locations.
Source: Suter (2002) (192K)

8 - Temperatures measured englacial at Colle Gnifetti, Monte Rosa area from 1983 to 2000 showing conditions for 1983, 1991, 1999, 2000.
Source: Suter (2002)

 

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29 August 2011
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